1. Field of the Invention
This invention relates to a method for the rapid heating and annealing of material comprising a ferrous or non-ferrous metal and is particularly useful where the material is of small cross-section or a shallow surface layer is to be treated.
2. Description of the Prior Art
Previous attempts to increase the heat transfer coefficient, when heating materials, have disadvantages. Salt baths have the disadvantage that salt is discharged together with the material treated, while in addition the heat transfer coefficients are limited to about 2000 kcal./m.sup.2.h..degree.C.. The customary metal baths also have the disadvantage that the metal is discharged together with the material heated. Furthermore, metal baths can usually be used only in a continuous process. If, for example, entire wire coils were to be dipped into a metal bath, the bath would solidify in the interior of the wire coil and the high heat transfer coefficient would be lost. The consequence is a very irregular rise in temperature and thus an irregular heat treatment.
The prior art discloses a method for the continuous heat treatment of strips, wherein not only is a metal bath used, but, in addition, sodium metal vapor is used in a middle region of a furnace for the purpose of heating to 882.degree.C. This method constitutes an improvement and is to be preferred for strip. It is true that the specification also recommends the continuous treatment of wire in the same way. This procedure entails disadvantages in the case of wire, as only thin wires can be passed effortlessly through the plant. Thicker wires require a bending device before each change of direction and this solution must be immediately rejected. Furthermore, in the case of wire the discharge of metal and consequently the expense in respect of the water baths which have to be connected for the purpose of removing the discharged metal and in respect of the drying device is extremely high. In addition, run-off crowns and welding devices are required upstream of the plant and shears and reels on the downstream side.